Medicare pays doctors according to a set schedule for each procedure. When a doctor treats a patient on Medicare he accepts assignment. In other words he agrees to accept what Medicare pays. For example, a doctor may normally charge $1000 for a procedure. Medicare may only approve $500. The doctor has agreed to accept that $500. Medicare will then pay 80% of the $500 and the patient pays 20%. The doctor is not allowed to bill for the balance.
Most providers accept assignment which makes it easier on them and you.
When your medical provider accepts assignment you can rest assured you will not be balance billed for excess charges.